Obesity is an established risk factor for a number of human diseases such as diabetes, heart disease, and cancer. Recent rapid increase in childhood obesity suggests the adverse effects of obesity on human health will be a major concern for the near future. To evaluate the impact of obesity on health risks, we first apply existing nonlinear optical (NLO) imaging technologies to characterize obesity-associated diseases in live tissues. Second, we develop novel NLO imaging technologies to advance the capability for diagnosis of obesity-associated diseases in live animals. Using a multi-modal NLO microscopy and an obesity- associated breast cancer rat model, we demonstrated the ability to image tumor cells and the significant components of the tumor stroma, such as blood capillaries, adipocytes, and collagen fibrils without labeling in live tissues. We observed that obesity alters the structure and composition of mammary tumor stroma and increases the incidence of mammary tumor aggressiveness. During this research training period, we will correlate the structure and composition of tumor stroma with tumor aggressiveness to lay the foundation for the development of NLO imaging as a diagnostic tool for breast cancer phenotype. A video-rate NLO microscope will be constructed to enable monitoring tumor progression in live animals. Using NLO microscopy on a diabetes-induced coronary artery disease pig model, we described the ability to visualize the significant components of an atheroma, such as intimal fatty streaks;foamy macrophages, collagen fibers, elastin, and cells lining the arterial wall without labeling in live tissues. We observed significant differences in the composition of atheromas in pigs with and without exercise therapy. During this research training period, we will further identify the components of atheromas and characterize the three-dimensional structural organization of these components. In addition, we will develop a catheter-based, multi-modal NLO endoscope that enables imaging of atheromas in situ. Our efforts in characterizing the composition and structural organization of atheromas and mammary tumor will facilitate rapid clinical application of novel NLO technologies to imaging and diagnosis of obesity-associated diseases. Obesity is an established risk factor for a number of human diseases such as diabetes, heart disease, and cancer. To evaluate the impact of obesity on health risks, we first apply existing nonlinear optical (NLO) imaging technologies to characterize obesity-associated diseases in live tissues. Second, we develop novel NLO imaging technologies to advance the capability for diagnosis of obesity-associated diseases in live animals.